The relative positional relation between an optical system with an objective lens and a medium may vary along the direction of the optical axis of the objective lens; the medium stores information by the action of a ray bundle condensed by the objective lens. A displacement detection device is used to monitor displacement of such a medium. When the displacement in the direction of the optical axis of the objective lens is monitored, servo light (measurement light) is introduced onto the optical path of the objective lens. Reflected light from the medium is then guided to the displacement detection device. Based on information on the displacement detected by the displacement detection device, defocusing is compensated for.